I know there's a lot of theme hate here, but if you make absolutely sure it is the RIGHT system menu and RIGHT region you will be ok. Still, I wouldn't do it unless you had bootmii as boot2. I have changed themes several time and never bricked, but I made sure it was the right sys menu and region. Someone on IRC recently asked me how to save his wii from a theme brick because he installed a 4.2U theme on a 4.2E Wii.
Wii64/WiiSX Hard Drive Fix
4.2u/ bootmii-boot2/ cfg usb loader v60/ Hermes Uloader 5.1D (for WW/VC)/ IOS 249 Rev 17/ IOS 222-223 v4/ IOS 224 v5.1/ 320gb Transcend HDD/ Darkwii System Menu Theme
I agree with Krafter in that the above statement isn't always true. The guys/gals who code these themes don't always know what they're doing. I've bricked multiple times off a bad theme (right off the wiibrew site as one example). Even changed firmware from 3.2->4.0->4.1->4.2 (along with the appropriate sysmenu files, obviously) to experiment and see if they wrote down the wrong number on the download page. I also have a knack for following directions to the letter, so I can certainly attest against user error. Just a bad theme.if you make absolutely sure it is the RIGHT system menu and RIGHT region you will be ok.
A brick off a bad theme has always been recoverable for me from Bootmii/boot2, but I'd never chance it on a Bootmii/IOS Wii even if I knew the theme worked ten times over. Ain't worth the hassle if it goes south.
"I think that the Wii is a beautiful piece of hardware, and a broken Wii is a tragedy. It doesn’t matter why or how." -- Bushing
if I were to brick with theme then I would get a new one for free thanks to gamestops warrenty polocy.